I wouldn't say a complete waste of time and money. It's just another proof of citizenship and it creates a record of your child citizenship with USCIS that might come handy when your kid gets picked up close the the US border and deported for not having proof of citizenship on her or him
I agree that nowadays with the passport card/passport book you can use one of them to obtain the other in case one is lost. One could keep the passport card in a safety box and that should be pretty safe.
I am not sure whether the passport agency would issue a passport if the Green Card document expires. Perhaps they do. I don't know.
Bottom line, from a practical point of view nkm-oct23 is right, in particular with the passport book/passport card backing up each other in case of loss. However, I think the certificate of citizenship has some other advantages. No expiry date. Record of citizenship at USCIS.